When signals are transmitted in mobile radio systems, the data signal to be transmitted is modulated onto a radio-frequency carrier signal, with the two signals being mixed with one another. After the modulation process, the combined signal is transmitted via the transmitting antenna. During the subsequent demodulation process at the receiving end, the carrier signal must be removed once again from the received signal by mixing with an LO (Local Oscillator) signal which is associated with the carrier signal, thus resulting in the so-called base band signal. The precise frequency of the carrier signal, also referred to as the carrier frequency, is generally not known in the receiver in its initial state and must first of all be determined or estimated by suitable methods. In this case, a broad frequency range is generally predetermined, in which the radio channel to be detected and which is governed by the carrier frequency is located. The object of the frequency synchronization process in the receiver is now to estimate as accurately as possible the mid-frequency of the transmission channel, which corresponds to the carrier frequency.
The following criteria can be used as a quality measure for the estimation method during the estimation process:
a) the accuracy of the estimation and
b) the time required for an estimation process.
In this case, excessively large estimation errors generally have a significant adverse effect on the performance of the receiver; excessively long estimation times are generally the result of an unacceptable, very complex implementation, and delay the setting up of the connection.
In the methods which are known from the prior art for frequency searching, a multiple stage approach is chosen in order to achieve a predetermined accuracy. First of all, the carrier frequency of the transmitter is determined to a specific accuracy by trying out all of the possible carrier frequencies fk, where k=1 . . . N. To do this, a so-called RSSI (Radio Signal Strength Indicator) measurement is carried out in order to pass through the permissible frequency range with a step width Δ and the received signal power is measured over a broad bandwidth, that is to say with a reception filter having a broad bandwidth. That frequency fkmax at which the most power is received is in this case the closest to the actual carrier frequency, and thus represents a first estimate of the carrier frequency. The frequency uncertainty of the first estimate results from the magnitude of the frequency step width Δ. A more accurate, second estimated value of the carrier frequency is determined in a second step, by now measuring the spectral received power using a narrowband reception filter in discrete steps in a narrow frequency band around the frequency that was found in the first estimation process. The disadvantage of this method is that a wide frequency bandwidth (for example of 60 MHz) must be searched through with a frequency step width which is relatively narrow in comparison to this bandwidth (for example 1 MHz) in the first method step, and this involves a long search time. However, the frequency uncertainty range which is relatively large in comparison to the required resolution nevertheless then remains, which must then be investigated again in the second method step, with a narrowband search. Once again, this step is very time-consuming.